A gas turbine, in particular in a gas and steam turbine, is usually used to generate electrical energy. To increase the performance of the gas turbine and thus to achieve the highest possible degree of efficiency, efforts are made to achieve a particularly high temperature of the working substance on the inlet side of the turbine of e.g. 1200° C. to 1500° C. However, such a high turbine inlet temperature may entail material problems, in particular in relation to the heat resistance of the turbine blades and vanes.
In order also to be able to operate reliably at such a raised turbine inlet temperature for a long service period, cooling of high-temperature turbine parts, such as, for example, rotating and/or guide blades, is usually provided in modern gas turbines. To this end, a coolant, for example, cooling air is applied to these turbine parts. In particular, a partial flow of the compressor air supplied by the compressor of the gas turbine can be enlisted as cooling air. In order to be able to enlist this partial flow of compressor air, the temperature of which may exceed 400° C. depending on the operating mode of the gas turbine, as coolant for the gas turbine this partial flow is usually cooled to temperatures of, for example, less than 200° C.
Such coolant cooling of a gas turbine usually takes place in a coolant cooler assigned to the gas turbine, in which cooling of the coolant takes place via heat exchange. The coolant cooler designed as a heat exchanger to this effect can be designed secondarily as a low-pressure steam generator in which a flow medium evaporates and the steam thus generated is fed into the water-steam circuit of a steam turbine or is also supplied to a district heating network to recover the energy. Generators known as water pipe steam generators or flue pipe steam generators which produce saturated steam are used for this purpose.
Precisely in the design of gas and steam turbines, a particularly standard design objective is the achievement of an especially high level of efficiency when converting the energy content of a fuel into electrical energy. With regard to this design objective, the results achieved to date for the transfer of heat produced during the cooling of the coolant of the gas turbine into the water-steam circuit of an assigned steam turbine have been only limited. To increase the level of efficiency attainable when using the heat produced during cooling of the coolant of the gas turbine, combined solutions with a two-stage coolant cooler were also taken into consideration in which both low-pressure and medium-pressure steam is generated during the cooling of the coolant. However, as has emerged, though slightly increased efficiency with regard to the use of heat produced during cooling of the coolant of the gas turbine is attainable in the case of such a solution, there is disproportionately high expenditure on plant technology.